3-1 CT1 - BENG101 Foundations of Biomedical Imaging Fall...

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BENG101 Foundations of Biomedical Imaging Fall 2009 X-ray Computed Tomography (CT) 1
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Introduction X-ray CT produces cross-sectional images representing attenuation of body Tomography – tomos (slice) graphy (writing) Thin x-ray beam scans a set of lines across the field of view, repeated for many angles, yields line attenuation measurements for all angles and distance from the center. This data can be “reconstructed” to yield the attenuation at each point in field of view. Parallel Beam Fan-Beam Geometries Repeated for many angles
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Introduction CT originally CAT (comput(eriz)ed axial tomography), reserved for X-ray CT. Reconstruction from “projection” data formulated by John Radon in 1917. Before this other tomography existed: Linear Tomography Source and Film move at constant speed in opposite directions. Patient plane (P1 – P2) always projected at same point on film, rest of body averaged out. Axial Transverse Tomography Film is horizontal and below focal plane. Patient & film rotate at constant speed, source is fixed. Focal plan stays in focus, other planes are averaged out.
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CT History First CT scanner (EMI) developed by Godfrey Hounsfield (1972). Based on math and experiments of Cormack in 1960s. (Shared Nobel prize 1979) First whole-body CT scanner ACTA ( automated, computerized, transverse axial scanner) Ledley in 1974 (Georgetown,DC). ACTA scanner (whole body) EMI scanner (head) Spiral CT (1989) and multislice CT (1998)
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CT Scanner
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Hounsfield Units Images consist of pixels (e.g. 512 x 512) representing the CT number (Hounsfield units) μ is Linear Attenuation Coefficient. Air = -1000 HU, Water = 0 HU, Bone > 400 HU. Large Dynamic range: tissue to bone or Small Dynamic Range: Soft tissue contrast Requires window/level operation 2 2 0 () . 1 0 0 0 0 H CTnumber HU H μ =
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X-ray Detectors in CT Scintillation crystal with PMT Used in earliest CT scanners (but low packing density) Gas Ionization Chambers X-rays ionize pressurized gas (xenon), free electrons and ions attracted towards anode and cathode to yield electrical signal. High packing density, but low quantum efficiency and speed.
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This note was uploaded on 04/30/2010 for the course BENG 101 taught by Professor Silva,g during the Fall '08 term at UCSD.

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3-1 CT1 - BENG101 Foundations of Biomedical Imaging Fall...

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